U.S. Pat. No. 7,640,106 is known in prior art describing an apparatus for determining the position of a selected object relative to a moving reference image, the apparatus including at least one reference frame transceiver assembly secured to the moving reference frame, at least one object transceiver assembly firmly attached to the selected object, an inertial measurement unit firmly attached to the selected object, an inertial navigation system (“INS”) secured to the moving reference image, and a tracking processor coupled with the object transceiver assembly, to the inertial measurement unit and to the inertial navigation system, the object transceiver assembly communicating with the reference frame transceiver assembly using magnetic fields, the inertial measurement unit producing IMU inertial measurements of motion of the selected object relative to an inertially fixed reference frame, the inertial navigation system producing INS inertial measurements of motion of the moving reference frame relative to the inertially fixed reference frame, the tracking processor receiving electromagnetic measurements resulting from the magnetic communication between the reference frame transceiver assembly and the object transceiver assembly, the tracking processor determining the position of the selected object relative to the moving reference frame by using the IMU inertial measurements and the INS inertial measurements to optimize the electromagnetic measurements.
French Patent FR2807831 is also known in prior art describing a device for measuring the position and orientation of a mobile object relative to a fixed structure, in a disturbed magnetic environment, including:                a first assembly of orthogonal coils emitting magnetic fields, secured to the fixed structure, defining a reference mark;        a second assembly of orthogonal coils receiving magnetic field(s), secured to the object, and forming a sensor, each of the coils belonging to a sensor channel.        
Such a device includes means:                for simultaneous and continuous field emission, on the coils of the first assembly;        for measuring, on the sensor channels, the vector sum of the emitted fields and of the disturbance fields generated by the environment;        for evaluating the disturbance fields;        for estimating fields emitted in an undisturbed environment by suppressing the evaluated disturbance fields from the vector sum; and        for computing the position and orientation of the object in the reference mark.        
U.S. Pat. No. 5,646,525 describes another example of equipment for determining the position and orientation of a helmet worn by a crew member in a vehicle including a generator, associated with the vehicle, which produces a rotating magnetic and electric field of fixed strength, the orientation and frequency within at least a portion of the vehicle. The apparatus also includes a plurality of detectors, each of which generates a signal proportional to at least one of the electric or magnetic fields, at least one point associated with the helmet and calculation circuitry responsive to the signal for determining the coordinates of the at least one point relative to the generator and for determining the position and orientation of the helmet.
U.S. Pat. No. 6,400,139 also describes an example of an apparatus for position/orientation tracking within a bounded volume. The methods and apparatus employ at least one fixed sensor, called a “witness sensor,” having a fixed position and orientation near or within the volume to account for electromagnetic distortion. One or more probe sensors are placed on an object having to be tracked within the volume, and the output of each witness sensor is used to compute the parameters of a non-real effective electromagnetic source. The parameters of the effective source are used as inputs for the computation of the position and orientation measured by each probe sensor, as if the object were in the non-distorted electromagnetic field produced by the effective source or sources. In addition to trackers for the helmet-mounted displays in aircraft, tank, and armored-vehicle applications, the invention finds utility in any electromagnetic tracking system that might be subject to electromagnetic distortion or interference.
In general, the solutions of prior art do not teach solutions to compensate for the disturbances not correlated with the transmitters (actual emitted fields).
U.S. Pat. No. 7,640,106 requires a first inertial sensor in the helmet and a second inertial sensor and an estimator (Kalman filter) for determining an orientation of an object. The solution requires providing a sensor on the fixed platform in order to determine the angular orientation of the helmet in the mark of the platform by incorporating the estimated relative velocity. Relative velocity is obtained by measuring the difference between:                the angular velocity of the mobile body measured at the output of an IMU angular velocity sensor attached to the mobile body, the orientation of which is to be determined, measured in a fixed inertial frame (inertial reference frame); and        the angular velocity of the inertial platform measured by an INS-type inertial unit.        
The solution, therefore, requires a double inertial system, doubling the noise and the errors.
Furthermore, the solution does not take into account the strong electromagnetic disturbances observed in a real cell, for example, a helicopter or airplane cell.
Furthermore, the solution requires an estimation to be carried out of the angular velocity.
The solution taught by U.S. Pat. No. 6,400,139 includes the interpolation of data coming from a plurality of sensors in view of creating a model of the fields sent by real sources, and modelling unknown or dummy sources to compensate the Eddy current disturbances. The solution consists of installing a plurality of fixed witness sensors in the vicinity of the volume in which the mobile body moves, in order to construct a model of the field measured by the witness sensors. The model is used for recomputing by interpolation the field measured by the sensor positioned on the mobile object, which does not make it possible to compensate the disturbance fields of Eddy currents, nor does it make it possible to process the disturbances of radiated and non-correlated disturbances (EMI). Only the ECI-type disturbances are correlated with the emitted radiative field.
All of the solutions of prior art require the use of an additional inertial platform to determine an additional mark in addition to the reference system provided by the inertial system of the aircraft, which complicates the implementation and the errors.